Ed Miliband is "struggling" in key areas of his role as Labour leader, Lord Mandelson has said.
The former business secretary, an architect of New Labour, said the leader faced the "unenviable" task of trying to redefine the party at the same time as fighting the coalition on the economy.
Asked about criticism of Mr Miliband's leadership, Lord Mandelson said he found it "tedious and boring".
He told BBC Radio 4's Today programme: "What I think Ed Miliband is doing, he's struggling with two things and they are not easy. One is that he is trying to oppose the Government on the economy where, legitimately or not, people will take different views.
"They think the Government's in the wrong place but in making an argument against what the Government is doing fighting the recession he is also sort of struggling with his own inherited legacy from the previous Labour government and they are not doing that easily or finely but nor is it simple to do.
"At exactly the same time he is struggling to invent a new left of centre political paradigm that isn't New Labour that, in a sense, takes lessons and experience from the last 15 years, not least from the experience of globalisation, revisit the issues to do with markets and inequalities and responsible capitalism, to sort of invent a new left of centre politics for the 21st century."
He added: "It's a rather unenviable job which I think he is doing well in the circumstances but it is not easy."
Lord Mandelson had easy praise, however, for his successors at the Department for Business, Innovation and Skills (BIS). He said: "If you see what (Business Secretary) Vince Cable is doing and what David Willetts in particular, the higher education and science minister, is doing - I read a speech of his the other day, I could have written every word of it myself - it's in a sense taking to a higher and more sophisticated level the sort of industrial policy thinking and actions that I was introducing when I came back to government.
"The only problem is this - it is done on such a ludicrously small scale."
Lord Mandelson said internationally the right wing had been "better at handling the rhetoric of austerity" and had tended to win the argument on "profligate" big government. The centre-left needs to fight back but "not by reverting to old arguments about state control and intervention".